(1) What prompted the searches for a planet beyond Neptune? Why did astronomers think there might be additional planets? Were their suspicions justified in the end? The earlier astronomers could not explain the orbits of Uranus and Neptune very accurately. They thought that there had to be a planet beyond Neptune that was perturbing the orbits. But it turned out that with better data and better calculations no "Planet X" is needed.

(2) Compare the albedo maps derived from the mutual eclipses with the maps derived from Hubble data. Did the mutual eclipse researchers get roughly the same features as shown by Hubble? (e.g., compare where the light and dark regions are on Pluto in each case). Well, there are light and dark patches. In fact it is not so bad - bright polar regions and dark patches near the equator. But the resolution is not very good in either case. We really do not know what the surface looks like.

(3) (a) HOW does Charon's orbit allow us to determine Pluto's density? We can use Netwon's Version of Kepler's Third Law - by measuring the orbital distance and orbital period of Charon, we can estimate Pluto's mass. The duration of the eclipses allows a measurement of Pluto's diameter - and hence volume. The density of Pluto is then the ratio of Pluto's mass to volume.

(b) What are the densities of rock and of pure ice? Rock has a density of about 3 g/cm3, ice is a little less than 1 g/cm3

(c) The density of Pluto is about 2 g/cm3--corresponding to about 70% rock and only 30% ice. By comparing the density of Pluto with the densities of Charon and of the moons of Uranus and Neptune, would you judge Pluto to have more or less ice that is typical for small bodies in the outer solar system? The densities of Charon and the icy satellites of Uranus (the densities of Neptune's small satellites are not known) are between 1.2 and 1.65 - much less than Pluto - suggestion more ice and less rock.

(d) Compare the density of Pluto with that of Triton. Triton has a similar density as Pluto - about 2 g/cm3. Also a little high - a little rock enhanced - than other small bodies in the outermost solar system.

(4) Compare the properties of Pluto and Charon listed above with the table of properties of terrestrial and giant planets, Table 8.2 of the textbook. Why Pluto is a "misfit" planet? Consider Pluto's size, orbit, moon, density, etc; When you consider Pluto's size then you have to say it is more like a terrestrial planet than the gas giants. But its lower density, inclined & eccentric orbit, its location in the outer solar system - then it really does not belong in the same catagory as Venus, Mars and Earth.

(5) (a) Say Pluto had a lot more ice before it suffered a big impact, what would have happened to the ice during the impact? Would the density of Pluto increase or decrease due to the impact? I would expect that much of the ice was vaporized during a big impact - this would make Pluto's density greater - more rock enhanced.

(b) What is believed to have happened to Triton that might similarly have led to Triton loosing ice and having a similar, more rocky composition as Pluto? Triton was captured by Neptune (it has a retrograde orbit - a sign of a captured moon). In the process of capture the moon Triton would have suffered severe tidal heating which could have driven off the volatile ices.

(6) (a) What are some of the criteria that people use for deciding whether to call Pluto a planet or not? I would say that the issue is whether it fits as anything else - it is not a moon, it is not a comet. It is big enough to pull itself into a sphere. It orbits the Sun. That means it has to be a planet - albeit a minor planet. If this means that the largest asteroids should also be called minor planets - that's fine too.

(b) Is this a scientific issue? Not really. Classification of objects is only useful when it serves a purpose of further understanding. Pluto is sufficiently different than the other planets that it is really not so important scientifically that it be called a planet - we know it is a Kuiper Belt object.

(c) So, what do YOU think? Should Pluto be called a planet? Of course it should!

(7) (a) Look at the plot above. The masses of outer solar system objects are plotted vs. distance from the Sun. The graph has scales that increase in powers of ten--with MASS (relative to earth) on the Y-axis vs. Distance from the Sun (in A.U.) on the X-axis. How many powers of 10 are there on each axis? In the vertial direction the mass ranges 15 powers of 10. In the x-direction there are 2 powers of 10 for the distance from the Sun.

(b) Note that the giant planets form a group that are 10-300 times the mass of the earth. Pluto, Charon and Triton form a group that is 0.001-0.01 times the mass of the earth--that is 0.1 to 1% of the earth's mass. The new "QB1" objects are about 10 to 100 times smaller than Pluto/Charon/Triton. The smallest objects, comets, are found outside Pluto's orbit, greater than 50 A.U.--how many Kuiper Belt objects (such as QB1) does it take to add up to the mass of the Earth? A typical mass of a KBO is 0.00002 Earth-masses. This means that you need 1/0.00002 = 50,000 of them to add up to an earth mass.

(c) How many comets does it take to add up to the mass of the Earth? Comets are the smallest objects on the chart - at 0.0000000001 times the mass of the Earth. So it would take 1010 of these objects to make up the mass of the Sun.

(8) Using this picture and what you know about the sizes of the orbits of the outer planets (e.g. Neptune's orbit is 30 AU from the Sun) how far away from the Sun are the objects at the inner edge of the Kuiper Belt? What about objects at the outer edge? The inner edge of the Kuiper Belt appears to be Neptune's orbit of 30 AU. There is no way of knowing how far they extend - 200-300 kilometers..